Pockets on Carradice are just stitched on and you can cut the stitching to remove the pocket. You may get a line of holes showing but these should seal up fade with use. I wouldnt recomend removing the pcokets, they are quite useful and accessable from the saddle.
You can add things as well but you need an industrial strength sewing machine to make much headway. The D rings on my panniers are attatched using loops of nylon webbing but a leather loop will do just as well.
Carrdice are ideal for people who abuse their bags, need a repairable bag or want to personalise them. The one thing you need to watch out for is creating stress raisers in any attachments.
I used a standard Carradice patch kit to repair some holes. My first patch had staight edges and created a line of weakness which would wear. I replaced it with patch having a wavy edge (like large pinking sissors) to distribute the stress.